Metal surfaces which are to be exposed to a corrosive environment are generally provided with a protective coating. The protective coating must be resistant to attack by the corrosive environment and must adhere well to the metal surface. If the corrosive environment is a high temperature corrosive environment, the protective coating must also be stable at the high temperature.
Automobile exhaust system components, e.g., a muffler, tailpipe or the like are subjected to a high temperature corrosive environment during operation of the automobile. This corrosive environment comprises acidic materials present in the exhaust gases which result from the operation of the catalytic converter which is part of the exhaust system. Automobiles employ a catalytic converter as part of the exhaust system to convert harmful substances produced by the internal combustion engine to harmless materials. One type of catalytic converter employed on automobiles comprises a honeycomb ceramic material coated in part with a catalyst capable of converting the substances which come in contact with it. In the catalytic converter, sulfur produced during burning of the fuel is converted to sulfur trioxide. The sulfur trioxide can adhere to the ceramic material and be retained for extended periods of time in the catalytic converter. This sulfur trioxide reacts with water vapor present in the exhaust system to form sulfuric acid, some of which may be carried out in the exhaust gases to deposit on the metal components of the exhaust system. Sulfuric acid itself if very corrosive to metal. Additionally, in rich burn (i.e., oxygen poor) engines, some of the nitrogen gas present in the intake air is converted in the catalytic converter to ammonia gas. The ammonia gas can combine with the sulfuric acid to form ammonium sulfate which can deposit on the components of the exhaust system. When the ammonium sulfate is exposed to water vapor which condenses on the exhaust system parts after engine shut down, a strongly acidic solution is formed which can cause substantial corrosive damage to the metal it contacts.
Certain high efficiency automobile engines produce relatively cool exhaust gases. This causes significant amounts of condensate to accumulate on exhaust system components like the muffler or tailpipes, especially during short trip, low speed driving. Since, in many cars, the muffler is placed relatively far away from the engine and catalytic converter, the muffler is the coolest component of the exhaust system. Thus the muffler provides surfaces on which the exhaust gases are most likely to condense, making the muffler particularly susceptible to corrosion.
In attempts to reduce the corrosive effects of exhaust gases on metal components of the system, various coatings have been suggested in patent literature. Pierson et al., in U.S. Pat. No. 4,416,920, teach that steel mufflers can be provided with an inexpensive, corrosion resistant coating of a lead-tin mixture. It is taught therein that the tin is necessary to alloy with the steel base and form a metallurgical bond between the coating and the steel base metal. In U.S. Pat. No. 4,330,598, Lee et al. teach providing a protective coating of zinc-aluminum to mild carbon steel surfaces, such as used in an automobile muffler, for improved resistance to corrosion and oxidation. Knight, in U.S. Pat. No. 4,074,010, teaches certain ceramic-paint coatings to be useful, for example, on mufflers to reduce corrosion thereof. Still further, in U.S. Pat. No. 4,042,426, Beiser teaches improving the corrosion resistance of a steel substrate, such as that used for manufacturing a muffler, by providing a corrosion resistant chromized coating thereon. Sasame et al., in U.S. Pat. No. 3,907,611, teach a method which comprises dipping a ferrous metal article, e.g., a muffler, in a molten metal bath containing aluminum or its alloy with chromium so as to improve the resistance of the ferrous metal to corrosion and oxidation. In still another attempt to reduce the corrosive effect of exhaust gases, mufflers have been fabricated of enamelled steel sheet metal as taught in U.S. Pat. No. 4,382,487 by Baumann. However, the fact that mufflers and tailpipes still need to be replaced on automobiles with great regularity attests to the fact that all of these proposed solutions have their deficiencies. Fethke et al., in U.S. Pat. No. 4,402,714, take yet another approach to solving the corrosion problem. They teach that corrosion of the metal parts of an automobile muffler can be significantly inhibited by placing an absorbent mass, preferably crystalline zeolitic molecular sieve, in the internal space thereof. The sieve is to be present in sufficient amounts to prevent condensation of water vapor from the engine exhaust gases on the walls of the muffler after engine shut down.
It is an object of the present invention to provide metal subtrates, such as those employed in an automotive exhaust system, with a coating which provides resistance to corrosion in a high temperature, corrosive environment. This object is realized by a method of the present invention which comprises providing a coating of a particular epoxy-amine composition on metal substrates, such as those employed in making articles for motor vehicle exhaust systems.
U.S. Pat. Nos. 4,517,321 and 4,608,404, to Gardner et al. and U.S. Pat. No. 4,579,885 to Domeier et al., which are commonly assigned, teach epoxy compositions comprising aromatic diamine hardeners, some of which are of the type which may be employed in the method of the present invention. The Gardner et al. compositions are taught to be useful to produce compositions which have improved tensile properties and high compressive strengths, i.e., when the compositions are combined with structural fibers. In the Domeier et al. patent, it is taught that the aromatic diamine hardeners must contain at least one alkyl substituent on an aromatic ring. It is further taught therein that a composition containing this specific type of diamine hardener, when used with epoxy compounds, exhibits reduced moisture absorption, improved solubility, and in certain cases, controlled reactivity which improves the processing characteristics of the composition. The compositions are taught to be useful with structural fibers to form composites. However, neither of these references teach employing the epoxy-amine compositions described therein as protective coatings for metal which is exposed to high temperature, corrosive environments.